1. Field of the Invention
The present invention relates to an electric vehicle having a plurality of motors, and a control device thereof.
2. Description of the Related Art
Conventional electric vehicles each having a plurality of motors and their control devices, are described in, for example, JP-A Nos. Hei 10(1998)-42408, Hei 2(1990)-179203, Hei 9(1997)-215108 and Hei 4(1992)-165901.
JP-A No. Hei 10(1998)-42408 has described a battery driven type forklift using AC motors as ones for driving and cargo-handling, and a DC motor as one auxiliary to steering. JP-A No. Hei 2(1990)-179203 has described a battery forklift using DC motors of the type used for driving, cargo-handling and steering assistance.
JP-A No. Hei 9(1997)-215108 has described a control device for an electric vehicle, wherein a plurality of VVVF (Variable Voltage Variable Frequency) inverters and CVCF (Constant Voltage Constant Frequency) inverters are connected in parallel with a power supply and capacitors are connected in parallel with the respective inverters, and even when the CVCF inverters malfunction and thereby the CVCF inverters are disconnected to substitute some of the VVVF inverters for the CVCF inverters, the electrostatic capacitance of each capacitor can be held in a manner similar to its pre-switching.
JP-A No. Hei 4(1992)-165901 has described a control device for an electric vehicle, wherein inverters and DC-DC converters are connected in parallel with a power supply through relays and capacitors are connected in parallel with the inverters and DC-DC converters, and timings provided to charge the respective capacitors are controlled according to timings provided to close and open the respective relays, thereby making it possible to perform mutual charging and discharging of the capacitors in equilibrium.
According to a device described in JP-A No. Hei 10(1998)-42408, since the AC motor is used as the motor used for the cargo-handling regardless of the non-need for complex control, the control device becomes complex in structure and thereby becomes expensive.
According to a device described in JP-A No. Hei 2(1990)-179203, since a DC motor is used as a motor for driving the wheels. However since the DC motor does not have a highly precise control, a delay time (time loss) caused at the time of changing into forward drive or backward drive increases, the feeling of torque escape (namely the feeling by which the torque is lost for a moment) caused at the time of transition from regeneration to powering increases, and a response at the time of brake regeneration or regeneration based on an acceleration switch-off goes down.
According to a device described in JP-A No. Hei 9(1997)-215108, closing/opening means having a switching contact must additionally be provided so that the electrostatic capacitance of each capacitor can be held in a manner similar to the pre-switching, and hence the control device becomes expensive. No allowance is made for the operation of the control device where any of the VVVF inverters connected in parallel with the power supply has been at fault, and hence the reliability of the control device is degraded.
According to a device described in JP-A No. Hei 4(1992)-165901, even if a malfunction occurs in any of respective circuits for three or more motors, there is no consideration for executing the proper operation of the remaining two or more circuits without being affected by such a defective circuit.
An object of the present invention is to provide a usable an economical electric vehicle, and to provide a high-reliable control device suitable for an electric vehicle.
A basic feature of the present invention is as follows. An AC motor is used as a motor for driving wheels, and DC motors are used as a motor for assisting the steering of each wheel and a motor for driving a forklift device. An AC motor capable of performing precise control may preferably be used for driving the wheels to improve tactile feel to a driver. On the other hand, since complex control is unnecessary for the motor for assisting or assisting the steering of each wheel and the motor for driving the forklift device, DC motors capable of serving as a drive system simple in structure and low in cost may preferably be used. According to the present invention, since the AC motor is used as the motor for driving the wheels and the DC motors are used as the motor for assisting the steering of each wheel and the motor for driving the forklift device, a drive system, which provides good feelings to a driver during running, and which is used for steering assistance and driving of the forklift device, can be simplified in structure and reduced in cost.
Another feature of the present invention resides in that a plurality of power converting circuits for respectively converting power supplied from a power supply and supplying the same to a motor for driving wheels, a motor for assisting the steering of each steering wheel, and a motor for driving a forklift device are connected in parallel with the power supply, and capacitive elements are connected in parallel with at least two of the plurality of power converting circuits. According to the present invention, since the plurality of power converting circuits are connected in parallel with the power supply, they can respectively be operated independent of one another. Further, even if a malfunction occurs in any of the plurality of power converting circuits, since the capacitive elements are connected in parallel with at least two of the plurality of power converting circuits, the remaining power converting circuits can properly be operated without being affected by such a defective power converting circuit.